In a general public key system, each user terminal for performing private communication generates its private key and public key in a pair, and communication is available only when an opponent terminal for performing private communication obtains a proper public key. At this time, a user who has obtained the public key of the opponent terminal for performing communication receives a signature for the obtained public key from a trusted third party and uses the same as a certificate of the public key in order to prove that the obtained public key is proper.
Since the public key certificate connects the public key with justification to the identity of its owner, the user terminal trusts the public key obtained from the opponent terminal by means of the public key certificate, thereby performing communication with the opponent terminal.
However, when the public key certificate is discarded, the certificate-based public key system should maintain a discarded certificate list for a certain period or continuously manage such a discarded certificate, which consumes management time and costs.